Sister Jogging

My sister and I live together in a rented house pretty near to where we work. We have for about 5 years now. It's easy to share with someone you get on well with and have admired all your life.
She's three years older than me, light brown hair, about 5'2 and we're both pretty fit as we have the basement turned into a gym and small office. The desk and computer are at the back of the room and the weights and stepper and tread mill are in the middle of the room.
Every day when I get home I work out for a while and surf the net for a while and like clock work sis comes down and does her work out which is mostly running and stepping. For a long time I never took any notice of her work out as she's normally in track suits althought I have taken her underwear and wanked with them more times than I care to remember.
About two months ago she walked down to the basement in a pair of bright green lycra shorts and a tight matching tank top. We said our hello's and she jumped onto the tread mill. HOLY SHIT in all my born days I have never seen an ass move like it. She had her back to me as she started to walk, I could see light between her lips with every stride. I could just feel my cock between them, within seconds I had my dick in my hand under the desk pumping like mad. I was done in about 30 seconds. No way could I let this go. With come covered shorts I headed for my room and got my camera. I spent the next half an hour wanking and taking pics of her and she never had a clue. She was only about 6 feet in front of my desk.
I'm pretty sure she was teasing me, we've caught each other masterbating a lot of times. She knows I take her underwear because she has taken them back from my room to wash them when she needed them. She never says a word about and there's just a smile when we catch each other at it. We use the same computer and she knows the sites I look at. SIS IF YOU READ THIS PLEASE LET ME KNOW WHAT YOU THINK. I'LL STOP IF YOU ASK